In recent years, in order to control a PM motor at high accuracy, motor constants such as an armature resistance and an inductance of the motor must be accurately comprehended. For example, in position sensorless control in which a magnetic polar position of the PM motor is controlled without a sensor, many methods of estimating magnetic polar positions by using motor constants are employed. In vector control of a PM motor, current control is popularly employed, and motor constants are used to properly set a gain of a current control unit.
Thus, some techniques that easily require motor constants of a PM motor without rotating the motor are known (for example, refer to, PTL 1). A technique disclosed in PTL 1 requires a value of a wire wound resistor based on an input voltage and an input current obtained when a DC current is caused to flow in a PM motor. Fundamental wave components of an input voltage and an input current obtained when an AC current is caused to flow in a PM motor are extracted, and an inductance is calculated based on the magnitudes of the input voltage and the input current and a phase difference between the input voltage and the input current. In this manner, motor constants of the PM motor are calculated.
Another technique that calculates motor constants of a PM motor is disclosed in, for example, PTL 2. According to PTL 2, voltages obtained when DC currents having different high and low levels are caused to flow are stored. A difference between the stored voltages having the two levels is divided by a difference between currents having two levels to calculate a value of a wire wound resistor. At the same time, a time until a current obtained when a voltage based on a DC current at a high level is sharply changed into a voltage based on a DC current having a low level changes into a predetermined value is measured. Based on the measured time and the calculated value of the wire wound resistor, an inductance is calculated.
However, in PTL 1, in order to calculate the value of the wire wound resistor, a time until a DC current caused to flow in the PM motor is set in a steady state is required. Since the value of the wire wound resistor and the inductance are measured by using different test signals, long times are disadvantageously required for the measurement. Furthermore, detection accuracy of a phase difference is deteriorated unless a frequency of an AC current caused to flow in order to calculate an inductance is properly set. For this reason, an error of the measured phase difference becomes large.
On the other hand, also in PTL 2, like in PTL 1, in order to calculate a value of a wire wound resistor, a time until a DC current caused to flow in a PM motor is set in a steady state is required. Since the value of the wire wound resistor and the inductance are measured by using different test signals, long times are disadvantageously required for the measurement. Furthermore, when a time until a current flowing when a voltage having a high level is sharply changed into a voltage having a low level changes into a predetermined value is measured, determination of a level and a measured time easily include errors. In a motor in which inductances change depending on the magnitude of a current, since a response does not have a predetermined time constant, an accurate inductance cannot be calculated.    PLT 1: Unexamined Japanese Patent Publication No. 2000-312498    PLT 2: Unexamined Japanese Patent Publication No. 2009-232573